Extensometer



April 20, 1965 J. P. DE NICOLA 3,178,827

EXTENSOMETER Filed 001;. 24, 1962 United States Patent Office 3,178,827 Patented Apr. 20, 1965 3,178,827 EXTENSGMETER Joseph P. De Nicola, 249 E. Squanturn St., North Quincy, Mass. Filed Oct. 24, 1962, Ser. No. 232,764 4 (Ilaims. (Cl. 33-147) This invention relates to extensometers, and more particularly to such devices especially useful in the testing of low modulus materials.

It is a primary object of the present invention to provide an extensometer which is so simple and light it may be mounted directly on a specimen, which provides for great ease of operation, and which permits measurement of very large amounts of stretch or extension without interference between the extensometer and the stress-transmitting specimen main grips above and below the extensometer. Another object is to provide such an extensometer in which successive amounts of strain are signalled with accuracy and sophistication. Yet another object is to provide such an extensometer in which the pattern of signals may be changed easily if desired, for a dif ferent character of specimen or otherwise.

Generally speaking, the invention involves an extensometer with an upper grip, a lower grip, and a flexible elongated member anchored in one and drawn through the other, with the latter of which it cooperates to signal how far apart the two extensometer grips are at predetermined times. In preferred embodiments the upper and lower extensometer grips carry cooperating alinement and integrating means. In other preferred embodiments, the flexible member is yieldably anchored only in the grip relative to which it does not move in ordinary use, to minimize any possibility of flexible member breakage in use, owing to twisting or other effects upon stressing a specimen to rupture.

Other objects, advantages, and features including the configuration and arrangement of parts will appear from the following description of the presently preferred embodiment of the invention, taken in conjunction with the attached drawings thereof, in which:

FIG. 1 is an isometric View, from the front, of the extensometer mounted on a specimen;

FIG. 2 is a rear elevation of the extensometer, with the lower portion of its flexible elongated member broken away;

FIG. 3 is a sectional view taken at 33 of FIG. 2;

FIG. 4 is an isometric view, partially in section and partially broken away, of the flexible elongated member; and

FIG. 5 is a plan view of the extensometer.

Referring now with more particularity to the drawings, there is shown in FIG. 1 a specimen S of rubber which is held between a pair of stress-transmitting grips G. Carried by the specimen is an extensometer indicated genorally at E.

The extensometer E includes an upper extensometer grip 10, a lower extensomer grip 12, and a flexible elongated tape 14.

The upper grip includes a grooved support 16, a tongued closure 18 secured thereto by screws 24) to define therewith a slot through which the elongated tape 14 just freely moves, a fiat spring 22 held against the tongued closure 18 by screw 24, and with its contact point 26 biased through mating hole 28 in tape 14. Mounted on pivot 30 in back of the support 16 is a clamp member 32 with a flat handle portion 34 opposed to the thin flange 36 of support 16, and with a knife edge element 38 carried thereon on pivot 40. The support 16 and closure 18 are preferably of plastic, for example Iexan, Delrin, or Teflon. The clamp member and knife edge element are preferably stainless steel.

The lower grip 12 includes a grooved support 42, a tongued closure 44 secured therein by screw 20 and screws 46, and a pair of flat spring contacts 48 with semicylindrical contact surfaces 49 held by members 50 in closure 44. Mounted on -a pivot back of the support 42 is another clamp member 32 with flat handle portion 34 opposed to the thin flange 52 of support 42. Both clamp members 32 have their correspondingly arranged knife edge members 38 urged against the respective supports by corresponding springs 54.

Extending downwardly from the support 16 are a pair of parallel pins 56 which removably seat in a pair of parallel holes in the support 42. These cooperating means provide for alinement of the upper and lower extensometer grip at the beginning of a test, as well as operatively integrating them at this stage so that both handle portions 34 may be easily squeezed toward the portions 36 and 52 simultaneously with one hand, so that the extensorneter may be mounted with ease and accuracy.

The flexible elongated member 14 comprises a length of tape which is of uniform thickness, .008 inch in the preferred embodiment, throughout. In the preferred embodiment it includes a plastic body portion 60 which acts as an insulator, and is preferably of Aclar, in which is embedded a ladder 62 of copper about .003 inch in thickness. As shown in FIG. 4, one surface of the ladder 62 is coplanar with a surface of the plastic insulating body 60, and the said surface of the latter is accordingly interrupted by both the longitudinal portions 62a and the transverse portions 62b of the ladder. The other surface of the body 60 is of course uninterrupted, the thicknesses of plastic of the body and of the ladder being such that the latter does not extend quite half way through the former. The ladder configuration has structural integrity and tends to prevent twisting or warping of the elongated member 14. The smooth coplanar surface along which centrally of the tape 14 the surfaces of the copper rungs alternate with rectangles of plastic surface makes possible smooth movement thereover of the contact point 4? of the spring contact 48 on the right as seen in FIG. 2, so that bouncing of the contact point 49 is avoided and precise and sensitive signal outputs are achieved.

At its upper extremity the tape 14 is centrally longitudinally slotted to provide a pair of handles 64 adapted to move by the detent 26 on each side thereof so that the end of the strip 14 and said handles may be easily threaded and pushed through the slot defined between support 16 and closure 18, with the handles 64 appearing above the upper grip 10 before the unslotted part of tape 14 reaches detent 26. The handles 64 may then be grasped and the strip pulled along, its portion above hole 28 raising the detent as it passes therebeneath, and the detent finally seating in hole 28 of strip 14, against which it is urged by spring 22 with force suflicient to hold strip 14 fixed in grip 10 during measurement of extension of specimen S, but insufficient to prevent the strips being pulled out of grip 10 without injury if any unusual force is exerted thereon, as upon a specimens breaking.

The contact point 49 of the contact 48 on the left as viewed in FIG. 2 is at all times over the longitudinal portion or leg 62a to the left of the strip 14 as viewed in FIG. 4.

In operation, strip 14 is inserted upwardly through lower grip 12 and then through upper grip 10, spaced away from grip 12, until handles 64 appear above grip 10. The strip is then pulled upwardly by the handles until detent 26 seats in hole 28. The lower grip 12 is then moved toward the upper grip l0, engaging pins 56 in the latter in the mating holes of the former, until the grips are in abutting contact. The handles 34 and 0pposed portions 36 and 52 of the two grips are then simultaneously squeezed between a thumb and a finger,

Q3) moved over a specimen, and released to clamp the specimen therebetween. By virtue of the pivoted knife edge arrangements, parallel line clamping with even force distribution is achieved on each side of the specimen. The lower grip G is then lowered relative to the upper one. Both the upper and lower grips 10 and 12 also move downwardly, but at different rates, the difference corre sponding to the rate and amount of stretch occurring between them (at the knife edges thereof). This difference causes the grips 10 and 12 to move apart, pulling strip 14 longitudinally through grip 12. The left contact point 49 is constantly in contact with leg 62a of copper ladder 62, and when the other contact 49 engages a rung 62b, a circuit is completed between the two contacts 48 to give a signal, which may suitably be a jog-and-return movement in the X direction of the needle on a chart recording load-time as XY, to record time after which and load under which each amount of stretch has occurred. If the test is continued to specimen breakage, the strip 14 may if subjected to any serious force, pull completely free from either grip iii or grip 12, to minimize any possible injury to it.

Other embodiments of the invention within the following claims will occur to those skilled in the art. For example, a longitudinally extending resistance wire may be embedded in a plastic strip, with one contact engaging it at the upper extensometer grip and one at the lower, the resistance Wire completing a circuit therebetween and the amount of resistance therebetween being measured to.

give the distance at any given instant therebetween. In this embodiment, the resistance figure, reflecting strain, may be used to drive a chart in the Y direction, to obtain a stress-strain XY curve directly.

I claim:

1. An extensometer comprising a first grip including a clamp member for gripping a specimen to define a first bench mark, a second grip including a clamp member for gripping said specimen at a second bench mark spaced from the first, a flexible elongated strip anchored for measuring use in said first grip and longitudinally movable in measuring use relative to said second grip, said strip including insulating portions and electrically conductive portions with surfaces in the same plane, and a pair of contacts carried by at least one of said grips for engaging said electrically conductive portions and completing a circuit therebetween, at least one of said coniii tacts being mounted for sliding movement along said surfaces and said plane.

2. The extensorneter of claim 1 in which said electrically conductive portions are a ladder of metal with 1ongitudinally extending legs and transversely extending rungs, said legs and rungs having a coplanar surface in said plane and said insulating portions are plastic of greater thickness than said ladder and in which said ladder is embedded, and in which both said contacts are mounted on said second grip.

3. The extensometer of claim 1 in which said strip is rclie ed at an end thereof by a longitudinal blind slot forming handles on both transverse sides thereof and includes a hole inwardly longitudinally spaced from said slot, in which a spring-biased detent mounted on said first grip extends into said hole to yieldably anchor the same relative to said first grip, said detent being narrower in a transverse direction than said slot, and said handles extending substantially above said first grip.

4. The extensorneter of claim 2 in which said strip is relieved at an end thereof by a longitudinal blind slot forming handles on both transverse sides thereof and includes a hole inwardly longitudinally spaced from said slot, in which a spring-biased detent mounted on said first grip extends into said hole to yieldably anchor the same relative to said first grip, said detent being narrower in a transverse direction than said slot, and said handles extending substantially above said first grip, in which said grips include mating integrating alinement portions, and in which each said grip includes a clamping member with a specimen engaging portion and a clamp handle and a support with a portion opposite said clamp handle, said clamp handles being sized and positioned to permit simultaneous squeezing of both said clamp handles toward both said portions opposite thereto between a single thumb and a single finger to open both said grips for mounting said extensometer on a specimen.

References t'lited by the Examiner UNITED STATES PATENTS 1,139,489 Church May 18, 1915 1,679,751 Stevenson Aug. 7, 1928 2,560,135 Shearer July 10, 1951 2,833,045 Strimel May 6, 1958 ISAAC LISANN, Primary Examiner. 

1. AN EXTENSOMETER COMPRISING A FIRST GRIP INCLUDING A CLAMP MEMBER FOR GRIPPING A SPECIMEN TO DEFINE A FIRST BENCH MARK, A SECOND GRIP INCLUDING A CLAMP MEMBER FOR GRIPPING SAID SPECIMEN AT A SECOND BENCH MARK SPACED FROM THE FIRST, A FLEXIBLE ELONGATED STRIP ANCHORED FOR MEASURING USE IN SAID FIRST GRIP AND LONGITUDINALY MOVABLE IN MEASURING USE RELATIVE TO SAID SECOND GRIP, SAID STRIP INCLUDING INSULATING PORTIONS AND ELECTRICALLY CONDUCTIVE PORTIONS WITH SURFACES IN THE SAME PLANE, AND A PAIR OF CONTACTS CARRIED BY AT LEAST ONE OF SAID GRIPS FOR 